Yarn feeding devices of the above-indicated kind are used for positively feeding a yarn, for example, to a knitting station of a circular knitting machine. The rotatable drum is driven in synchronism with the operation of the circular knitting machine. Thus, the yarn supply to the knitting station also takes place in synchronism with the operation of the knitting machine. The yarn detector element usually has the form of a pivotable detector arm lying against a portion of the yarn between the yarn-braking unit and the rotatable drum. As long as no yarn breakage occurs and the yarn has its normal tension, the detector element has a first position. In case a yarn-breakage occurs or the tension of yarn falls essentially below the normal tension level, the detector element pivots around its axis and thereby operates a switch which is usually connected to a stop-motion-relay for terminating the operation of the knitting machine.
A prior art yarn feeding device of the above-mentioned kind is, for example, previously known from the references GB-A No. 2 026 554 and GB-A No. 2 027 456. At the prior art yarn feeding device as known from the first-mentioned reference, the detector element has the form of a pivotable sensing arm arranged between the yarn-braking unit in the form of a disc-brake and said rotatable drum. A yarn-guiding hook fixedly secured to the yarn feeding device is arranged between the detector arm or sensing arm and the rotatable drum so as to define a surveyed portion of the yarn between the disc-brake and the guiding hook. The sensing arm lies against the yarn due to its own weight. In case of a yarn-breakage, the sensing arm or detector arm of the prior art feeding device pivots downwardly and thereby actuates the stop-motion-relay which, in turn, immediately interrupts the operation of the circular knitting machine. Although yarn feeding devices of this type have proven to be reliable when processing normal yarns, problems occurred when feeding so-called "lively" yarns to the knitting machine. "Lively" yarns are, for example, twisted yarns which show a peculiar lively behaviour when feeding them. It has turned out that undesirable vibrations of the detector arm may occur during the feeding of these "lively" yarns resulting in an erroneous operation of the stop-motion-relay actuated by the detector arm although the average tension of the yarn has not fallen for a longer period of time below a critical value and although no yarn-breakage has occurred. In other words, the detector arm of the prior art yarn feeding device erroneously actuates a switch associated thereto, causing a fault indication, although no fault which is detrimental to the operation of the knitting machine, has occurred.
U.S. Pat. No. 3,937,417 discloses a yarn tensioning apparatus for generating a generally uniform output tension level of the yarn leaving the apparatus. It comprises an input roller, an output roller, a friction pin and a friction nose lying against the friction pin, which friction nose is pivotably arranged in the yarn tensioning apparatus. The pivotable friction nose is adapted for compensating for varying yarn tensions by increasing or decreasing the wrapping angle of the yarn with regard to the friction pin.
In view of the above prior art, the present invention is based on the object to accomplish a yarn feeding device of the above-indicated kind having an improved reliability in detecting feeding faults like insufficient yarn tension or yarn-breakage whilst preventing erroneous fault detections.
In accordance with the present invention, the detector element fulfills two tasks at the same time, namely the surveying of the tension of the yarn or the detection of possible yarn-breakage and a dynamic controlling of the brake force which is exerted by the yarn-braking unit on the yarn guided through it. The detector element compensates for a reduction in the yarn tension by increasing the wrapping angle of the yarn passing the yarn-braking unit. It has turned out that this unique double-function of the detector element of the claimed yarn feeding device guarantees an optimal dynamic behaviour of the detector element and of the yarn even when processing a so-called "lively" yarn. In addition, this yarn feeding device accomplishes an essentially constant yarn tension in the yarn fed to the rotatable drum, which prevents the yarn from losing contact with the braking unit, which problem occurred at the prior art yarn feeding device.